Spectators watched late Thursday afternoon as the adult attacked the 3-month-old female, but workers could not intervene because it is not safe.

Lewis added that according to visitor reports, the adult chimp grabbed the baby from her mom, Gracie, "really fast" at around 3:30 p.m. One observer said that the attacker, who was not the father, threw the baby around after snatching her.

The zoo had been introducing the baby chimp gradually into the exhibit, and to some staff the incident came as a surprise.

Hear Becker describe how zoo staff reacted during the incident and see footage of the baby before it was killed:

Zoo officials apologized for the chimpanzee's unexpected actions, but added that the animals can sometimes exhibit aggressive and violent behavior. Lewis told NBC LA he talked to several visitors to help them process what they had seen.

"I felt bad for the people that saw that happen today. There were young children there that were so sad, you know, it's just really heartbreaking," he said.